In an offshore drilling operation, a drillstring extends from a rig into a wellbore whereat it drills deeper and deeper into the sea floor. The drillstring extends through a riser which reaches from the rig platform to the wellhead, usually with a subsea blowout prevention stack between it and the ocean floor. During drilling, mud removed from the wellbore is drained to the surface through the riser. The riser can be over 30 cm wide (e.g., it can have about a 53 cm diameter) and it may be thousands of meters long. The riser is typically made of a succession of riser pipes having adjacent ends that are connected and disconnected on the rig to raise and lower the riser into the ocean.